Respiratory System for Inducing Therapeutic Hypothermia

ABSTRACT

The present invention provides a method and apparatus for controlling a patient&#39;s body temperature and in particular for inducing therapeutic hypothermia. Various embodiments of the system are described. The system includes: a source of breathing gas, which may be in the form of a compressed breathing gas mixture; a heat exchanger or other heating and/or cooling device; and a breathing interface, such as a breathing mask or tracheal tube. Optionally, the system may include additional features, such as a mechanical respirator, a nebulizer for introducing medication into the breathing gas, a body temperature probe and a feedback controller. The system can use air or a specialized breathing gas mixture, such as He/O 2  or SF/O 2  to increase the heat transfer rate. In addition, the system may include an ice particle generator for introducing fine ice particles into the flow of breathing gas to further increase the heat transfer rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/587,103, filed Jul. 9, 2008; which is the National Stage ofInternational Application No. PCT/US2005/002600, filed Jan. 24, 2005;which application claims the benefit of U.S. Provisional Application No.60/538,789, filed Jan. 22, 2004. These applications are incorporated byreference in their entirety as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates generally to apparatus and methods forselective modification and control of a patient's body temperature. Moreparticularly, it relates to a respiratory system and methods for raisingand lowering a patient's body temperature by heat exchange with thepatient's lungs. The respiratory system provides rapid induction oftherapeutic hypothermia by having the patient breathe a respiratory gasthat carries with it ice particles or a frozen mist to enhance heatcapacity. The respiratory gas may be air or a special gas mixture thatincludes oxygen (about 20% concentration or more) and a gas with a highheat capacity (Cp) for more effective heat exchange, such as helium orsulfur hexafluoride.

BACKGROUND OF THE INVENTION

The respiratory system of the present invention is useful for treatingpatient's with hypothermia or hyperthermia and for inducing therapeutichypothermia for treating a variety of conditions, including acutemyocardial infarction and emergent stroke.

Man is considered to be a tropical animal. Normal functioning of thehuman animal requires a body temperature of approximately 37 degreesCelsius (98.6 degrees Fahrenheit). The body can self-compensate forsmall upward or downward variations in temperature through theactivation of a built-in thermoregulatory system, controlled bytemperature sensors in the skin. The response to an upward variation inbody temperature is the initiation of perspiration, which moves moisturefrom body tissues to the body surface. When the moisture reaches thesurface it evaporates, carrying with it a quantity of heat. Theexplanation for a person becoming thirsty when exposed to a hotenvironment for a period of time is that fluids lost due to perspirationmust be replaced. The response to a downward variation in bodytemperature is shivering, which is the body's attempt to generate heat.Shivering is an involuntary contraction and expansion of muscle tissueoccurring on a large scale. This muscle action creates heat throughfriction.

Hypothermia is defined as a core temperature of less than 35 degreesCelsius. Hypothermia is also considered the clinical state of subnormaltemperature when the body is unable to generate sufficient heat toeffectively maintain functions. Many variables contribute to thedevelopment of hypothermia. Age, health, nutrition, body size,exhaustion, exposure, duration of exposure, wind, temperature, wetness,medication and intoxicants may decrease heat production, increase heatloss, or interfere with thermostability. The healthy individual'scompensatory responses to heat loss via conduction, convection,radiation, evaporation and respiration may be overwhelmed by exposure.Medications may interfere with thermoregulation. Acute or chroniccentral nervous system processes may decrease the effectiveness ofthermoregulation.

Mild Hypothermia is when the core temperature is 34-35 degrees Celsius.The patient is still alert and able to help him/herself and intenseshivering begins. The patient's movements, however, become lesscoordinated and the coldness creates some pain and discomfort.

Moderate Hypothermia is when the patient's core temperature is 31-33degrees Celsius. Shivering slows or stops, muscles begin to stiffen andmental confusion and apathy sets in. Speech becomes slow, vague andslurred, breathing becomes slow and shallow, and drowsiness and strangebehavior may occur.

Severe Hypothermia is when the core temperature drops below 31 degreesCelsius. Skin is cold, may be bluish-gray in color, eyes may be dilated.The patient is very weak, displays a marked lack of coordination,slurred speech, appears exhausted, may appear to be drunk, denies thereis a problem and may resist help. There is a gradual loss ofconsciousness. There may be little or no apparent breathing, the patientmay be very rigid, unconscious, and may appear dead.

Simple methods for treating hypothermia have been known since very earlytimes. Such methods include wrapping the patient in blankets,administering warm fluids by mouth, and immersing the patient in a warmwater bath. Even these simple methods may be effective if thehypothermia is not too severe. These simple methods are limited in theireffectiveness however. Wrapping the patient in blankets ultimatelydepends on the patient's own production of heat to rewarm his body. Ineven moderate cases of hypothermia, or in the case of an ill or injuredpatient, the patient may simply be too weak or exhausted to producesufficient heat. Oral administration of a warm fluid requires that thepatient be conscious and capable of swallowing the fluid. Since loss ofconsciousness occurs early in hypothermia, this method is also limitedto moderate cases. Immersion of the patient in a warm water bath isoften simply impractical. For example, immersion of a patient undergoingsurgery would obviously be undesirable. Furthermore, the immersiontechnique is time consuming and may be ineffective in that it requiresthe transmission of warmth from the patient's skin surface into the bodycore before the benefit of the warmth can be realized. Other devicesallow for the direct warming of a patient's blood. These methods involveremoving blood from the patient, warming the blood in external warmingequipment, and delivering the blood back into the patient. While suchmethods are much more effective than any of the simple methodspreviously described, they are disadvantageous for other reasons. First,the apparatus involved is quite cumbersome. Second, some danger isinvolved in even the temporary removal of significant quantities ofblood from an already weakened patient. In fact, a further drop in bodytemperature is often experienced when blood is first removed for warmingin the external apparatus. Finally, special catheters are used for thedirect warming of a patient's blood. However, those catheters require atrained staff to insert the device to a central blood vessel of thepatient and those physicians are available only in specific units andnot in the ambulance or even not always in the emergency room. Thoseinstruments are also very expensive and thus are not available for everycaregiver.

Hyperthermia is a condition of abnormally high body temperature. It mayresult from exposure to a hot environment, overexertion, or fever. Bodycore temperatures can range from 38-41 degrees Celsius due to fever andmay be substantially higher in cases of exposure and overexertion. Likehypothermia, hyperthermia is a serious condition and can be fatal. Alsolike hypothermia, simple methods for treating hyperthermia, for example,immersion of the patient in a cool water bath or administration of coolfluids, have long been known. In general, it is as hard to treathyperthermia as it is to treat hypothermia.

Recent medical reports have described the use of controlled hypothermiaas a means to reduce oxygen consumption of tissue, such as the heartmuscle and the brain during decreased perfusion that occurs as a resultof myocardial infarction and ischemic stroke (respectively), which leadsto reduced damage and decrease of the infarcted area. Medical reportshave also described the prophylactic use of controlled hypothermiaduring cardiac surgery or interventional cardiology procedures forreducing damage from ischemia and/or embolization in the heart and brainduring and after the procedure.

The following patents and patent applications describe apparatus andmethods for affecting a patient's body temperature. These, and all otherpatents and patent applications referred to herein, are herebyincorporated by reference in their entirety.

WO03059425 Method for altering the body temperature of a patient using anebulized mist—Body temperature reducing method involves administeringnebulized mist at temperature below body temperature of patient untilpatient's temperature is reduced.

US20030136402 Method for altering the body temperature of a patientusing a nebulized mist—Body temperature reducing method involvesadministering nebulized mist at temperature below body temperature ofpatient until patient's temperature is reduced.

U.S. Pat. No. 6,303,156 Noninvasive method for increasing or decreasingthe body temperature of a patient—Increasing or decreasing bodytemperature for treating e.g. hemorrhagic shock comprises administeringoxygen and sulfur hexafluoride gas mixture by hyperventilation.

EP1089743 Composition containing sulfur hexafluoride and oxygen, forincreasing or decreasing the body temperature of a patient—Increasing ordecreasing body temperature for treating e.g. hemorrhagic shockcomprises administering oxygen and sulfur hexafluoride gas mixture byhyperventilation.

WO9966938 Composition containing sulfur hexafluoride and oxygen, forincreasing or decreasing the body temperature of a patient—Increasing ordecreasing body temperature for treating e.g. hemorrhagic shockcomprises administering oxygen and sulfur hexafluoride gas mixture byhyperventilation.

US20030066304 Method for inducing hypothermia—Hypothermia-inducingtreatment method for patient in cardiac arrest involves performingcontinuous administering of phase-change particulate slurry to patientin cardiac arrest until state of hypothermia is induced to patient.

U.S. Pat. No. 6,547,811 Method for inducing hypothermia—Improvement of acardiac arrest patient's outcome by pre-hospital administration of aphase-change particulate slurry internally until a state of hypothermiais induced.

WO0108593 Method for inducing hypothermia—Improvement of a cardiacarrest patient's outcome by pre-hospital administration of aphase-change particulate slurry internally until a state of hypothermiais induced.

US20030131844 Inducing hypothermia and rewarming using a helium-oxygenmixture—Composition useful for treating ischemic event by inducinghypothermia comprises a gas mixture comprising helium and oxygen havingtemperature significantly different than normal human body temperature.

WO03047603 Breathable gas mixtures to change bodytemperature—Composition useful for treating ischemic event by inducinghypothermia comprises a gas mixture comprising helium and oxygen havingtemperature significantly different than normal human body temperature.

U.S. Pat. No. 5,755,756 Hypothermia-inducing resuscitationunit—Hypothermia inducing apparatus for cardio pulmonary resuscitationin accident—has stretcher and liquid oxygen and carbon dioxide sourcesthat are maintained in movable state to transport patient from traumasite to hospital.

U.S. Pat. No. 6,149,624 Apparatus and method for the rapid induction ofhypothermic brain preservation—Assembly for inducing rapid hypothermicbrain preservation using a liquid pulmonary lavage, comprises a fluidreservoir, a heat exchanger for cooling the pulmonary lavage, a meansfor circulating the lavage and an effluent reservoir.

WO0018459 Mixed-mode liquid ventilation gas and heat exchange—Gas andheat exchange method in lungs for treatment of hypothermic pathologies,involves mixing oxygenated liquid comprising perfluorocarbon and gassuch as helium.

U.S. Pat. No. 6,582,457 Method of controlling body temperature whilereducing shivering—Method for controlling body temperature below setpoint, for reducing shivering, involves sensing temperature, generatingsignal, controlling temperature based upon signal and administeringagent.

U.S. Pat. No. 6,572,638 Method of controlling body temperature whileinhibiting thermoregulatory responses—Controlling body temperature whileinhibiting thermoregulatory response, involves controlling temperatureof patient's body using heat exchange device, and administeringanti-thermoregulatory response agent to the patient.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for controlling apatient's body temperature and in particular for inducing therapeutichypothermia. Various embodiments of the system are described. The systemincludes: a source of breathing gas, which may be in the form of acompressed breathing gas mixture; a heat exchanger or other heatingand/or cooling device; and a breathing interface, such as a breathingmask or tracheal tube. Optionally, the system may include additionalfeatures, such as a mechanical respirator, a nebulizer for introducingmedication into the breathing gas, a body temperature probe and afeedback controller. The system can use air or a specialized breathinggas mixture, such as He/O₂ or SF₆/O₂ to increase the heat transfer rate.In addition, the system may include an ice particle generator forintroducing fine ice particles into the flow of breathing gas to furtherincrease the heat transfer rate.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a first embodiment of the apparatus forinducing therapeutic hypothermia using a source of compressed breathinggas and a heat exchanger.

FIG. 2 is a schematic diagram of a second embodiment of the apparatusfor inducing therapeutic hypothermia using adiabatic cooling of acompressed breathing gas as an adjunct to the heat exchanger.

FIG. 3 is a schematic diagram of a third embodiment of the apparatus forinducing therapeutic hypothermia that includes a fluid source and afluid injector for creating ice particles or a frozen mist to enhanceheat capacity of the breathing gas mixture.

FIG. 4 is a bar graph showing the heat removed from the body (Watts) asa function of the rate of ice particles added to the breathing mixture(1/hr).

FIG. 5 is a bar graph showing a more detailed breakdown of the heatremoved from the body (Watts) as a function of the rate of ice particlesadded to the breathing mixture (1/hr).

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods and apparatus for modifying andcontrolling a patient's body temperature. According to the presentinvention the patient will be wearing a mask that will provide thepatient with the breathing mixture. Alternatively, the patient may beintubated with a tracheal tube. The system will work with patients whobreathe spontaneously as well as patients who are mechanicallyventilated. The respiratory gas may be air or a special gas mixture thatincludes oxygen (about 20% concentration or more) and a gas with a highheat capacity (Cp) for more effective heat exchange. The mixture can beregular or purified air, or air with a higher concentration of oxygen(from 20 to 100%). A different possible mixture will be oxygen andhelium, which has been proven to be safe and is used by divers and fortreatment of patients with airway disease such as asthma (for exampleHELIOX, which is 20% oxygen and 80% helium). The specific heat capacityfor helium is much higher than the specific heat capacity for air, thususing a helium/oxygen mixture will improve the heat flow rate and willenable a much more effective way of changing the patient's temperature.Alternatively or in addition, the mixture may include sulfurhexafluoride SF₆, which is a dense, nontoxic gas that has a much higherspecific heat capacity than air. The invention does not limit the gasmixture and other combinations of gasses that are biocompatible and safethat will serve for the temperature exchange may optionally be used.

Other gases may be added to the mixture. For example, carbon dioxide(CO₂) may be added to the gas mixture to help regulate the patient'srespiration rate. A CO₂ partial pressure will induce hyperventilation,i.e. cause the patient to breathe faster, which will increase the gasmixture flow rates and thus improve the rate of heat transfer within thepatient's lungs. Conscious patients may be asked to hyperventilate toincrease the gas mixture flow rate. Alternatively, patients may becaused to hyperventilate through use of positive and negative pressure,such as when a mechanical ventilator or similar apparatus is used. CO₂may be added to ensure that proper levels of CO₂ and O₂ are maintainedin the patient's blood. A higher concentration of CO₂ in the breathingmixture will help to prevent hypocapnia that may result fromhyperventilation. Other gases, for example nitrous oxide, can also beadded to the breathing gas mixture.

The invention will also enable controlling the pressure of the inhaledgas. Pressurizing the gas will further improve the gas mixture mass flowrate, and hence the heat transfer rate. The system will be able topressurize the inhaled gas to what is known to be safe to the patient(for example 1.5-2 atmospheres). Alternatively, the system may pulse thegas, i.e. vary the pressure continuously from high to low, which willhelp mixing the gas and improve the heat transfer rate.

Alternatively or in addition, jets of air (high pressure boluses of gas)delivered through the mask or through a tracheal tube will also helpmixing the gas and improve the heat transfer rate.

The invented device will also control the humidity of the inhaled gas.Changing the content of water in the inhaled gas could influence theheat flow rate.

The device will also record the patient's temperature using any knownway of measuring a patient's temperature (like a probe that will beinserted to the patient's rectum or a probe that will check thepatient's skin temperature but will be separated from the roomtemperature by a bandage that can isolate it effectively, or using IR tomeasure tympanic temperature or any other way to check temperature). Thedevice will use the recorded temperature as a feedback and will adjustthe temperature of the inhaled gas according to the desired patienttemperature.

The gas will be cooled or heated using any known way of cooling orheating. For example, a system of heat exchangers that will enable heatexchange between the gas outside the heat exchanger and liquid or gasinside the heat exchangers. Another option is to use an electricheater/cooler. Another option is to use a heat pump. Another means ofcooling the gas will be that the gas will be pressurized inside specialpressure resistant containers/bottles. When gas is released from a highpressure to a lower pressure heat is released and the temperature of thegas drops.

Since the rate of heat transfer is affected by the difference of thetemperatures of the gas and the patient, the device will be configuredto deliver gas at a very low temperature that will be proven to be safe.

The device will be used by the first aid giver, such as paramedics in anambulance or medical team outside the hospital, by a team in theemergency room or any other place where this treatment is necessary.Advantages of the system include ease of operation and the fact that itcould be operated with minimal training. Thus treatment of the patientcan begin much sooner after a heart attack, stroke or other eventcompared to other more invasive methods that must be performed in theemergency room or in the cath lab. Rapid treatment for these conditionshas been shown to improve patient outcomes by reducing ischemic damageand necrosis in the affected tissue.

The cold/hot gas will be in contact with the huge surface area of thelungs. The temperature of the blood in the lungs will change and thisblood will flow to the left heart and there will change the temperatureof the heart tissue. From the left ventricle some of the blood flows tothe coronary arteries (where it will continue to influence thetemperature of the tissue and change the metabolism and the oxygenconsumption). In the case of myocardial infarction, the effect of thischilled blood flowing directly into the coronaries is especiallybeneficial. The blood also flows from the left heart to the entire bodyand there it will also change the temperature as desired. In the case ofstroke, a portion of the cooled blood will flow to the brain, coolingthe tissue and reducing the metabolism and the oxygen consumption, whichwill reduce ischemic damage to the brain.

The system will potentially be able to use drugs like bronchodilatorsand local (inhaled) vasodilators or any other medications that willincrease the blood flow to the lungs for better heat transfer andprevent bronchoconstriction from the cold breathing mixture. The systemwill also potentially be able to be used in conjunction with drugs thatencourage perspiration, peripheral vasodilators and drugs that reduce oreliminate shivering. Other medications that can be administered byinhalation may be added to the breathing mixture, for example using anebulizer.

FIGS. 1-2 illustrate various embodiments of an apparatus for controllinga patient's temperature and inducing therapeutic hypothermia. Theseexamples are not intended to be limiting. The features of theseembodiments can be combined and arranged in other configurations to formother embodiments of the invention.

FIG. 1 is a schematic diagram of a first embodiment of the apparatus forinducing therapeutic hypothermia using a source of compressed breathinggas and a heat exchanger. The system includes a supply of compressedbreathing gas stored in a pressurized container 1. The gas is deliveredthough a heat exchanger 2 or other heating and/or cooling apparatus tothe mask 3 that the patient is using. Optionally, the system includes ahumidifier 8 to humidify the gas in order to improve heat transfer.

FIG. 2 is a schematic diagram of a second embodiment of the apparatusfor inducing therapeutic hypothermia using adiabatic cooling of acompressed breathing gas as an adjunct to the heat exchanger. The systemincludes a supply of compressed breathing gas 1 and a heat exchanger 2where the heat is exchanged with fluid located inside the heatexchanger. The temperature of the fluid in the heat exchanger is changedand monitored by a separate heating and/or cooling device 6. The systemincludes a device 5 that uses the cold temperature that results fromdepressurizing the gas to help cool the patient 5. A temperature sensorprobe 7 records the patient's temperature and a feedback controllerassociated with the heating and/or cooling device 6 uses it as feedbackto determine the desired temperature of the breathing gas.

FIG. 3 is a schematic diagram of a third embodiment of the apparatus forinducing therapeutic hypothermia that includes a fluid source 9 and afluid injector 10 for creating ice particles or a frozen mist to enhancethe heat capacity of the breathing gas mixture. The fluid source 9 willpreferably contain normal saline solution (0.9% NaCl) or any otherdesired solution, so that it will be isotonic with the patient's blood.Alternatively plain water, e.g. distilled water, may be used. If plainwater is used, NaCl may be added to the breathing mixture in the properamount to maintain an isotonic concentration or administered to thepatient orally or via another route. Optionally, the system may beconnected to a mechanical respirator 11, particularly for patients whoare not breathing spontaneously. The system may use air or one of thespecialized gas mixtures described above. The incoming breathing gas ispassed through a heat exchanger 12 to cool it to a temperature below thefreezing point of the injected fluid (below 0 degrees Celsius for waterand below −0.52 degrees Celsius for normal saline solution). The heatexchanger can utilize a refrigeration cycle, a reversible heat pump, athermoelectric heater/cooler, dry ice, liquid nitrogen or other cryogen,or other known heater/cooler to achieve the desired temperature.

A fine spray of fluid droplets is injected into the cooled breathing gasmixture to form a frozen mist of fine ice particles. The fluid injector10 may include an orifice-type atomizer or an ultrasonic atomizer toachieve a small and uniform droplet size. An ultrasonic atomizer willtypically produce droplets (and hence ice particles) with a size in therange of 2 to 5 microns, which can easily be suspended in the movingflow of the breathing gas mixture. However, larger or smaller dropletsand ice particles will also be effective. Optionally, the system mayinclude a screen or filter downstream from the ice particle generator tolimit the size of ice particles delivered to the patient. The amount ofice particles added to the breathing gas mixture is preferably in therange of 0 to 5 liters per hour (measured as the volume of fluidinjected to produce the frozen mist.) A flow rate of ice particles inthe range of 0.25 to 1 liters per hour is currently thought to besufficient for rapidly achieving hypothermia in an adult human patient.Due to the beat of fusion (the heat required to effect a phase changefrom liquid water to ice), the incoming breathing gas may need to becooled to a temperature significantly below the freezing point toachieve effective freezing of the fluid droplets. In addition, it may behelpful to pre-cool the fluid to a temperature close to freezing beforeit is injected into the breathing gas. An additional heat exchanger maybe included for this purpose. Optionally, the fluid injection can betimed with the pulsatile flow of breathing gas. Optionally, a fan 14 maybe included to constantly circulate the breathing gas within the systemto avoid the fluid droplets and ice particles from settling out of thebreathing gas. Preferably, the system is insulated 15 to avoidcondensation or frost formation on the exterior of the conduit and toprevent heat exchange with the ambient environment. Optionally, theinterior surface of the conduit may be coated with Teflon or ahydrophobic coating 16 to avoid fluid or ice from accumulating on theinterior surface. A drain 13 is provided for removing any fluid thataccumulates within the system. Optionally, the system may include anebulizer 18 for introducing medications into the flow of breathing gas.

Other methods may be used for adding the ice particles to the breathinggas mixture. For example, solid ice may be ground or shaved into smallparticles and added to the flow of the breathing gas mixture.Alternatively, small ice particles can be produced and stored ahead oftime and added to the flow of the breathing gas mixture, for exampleusing a screw-type metering device, a vibratory feeder, or any othermeans of controlling the quantity of ice particles delivered into thebreathing mixture. Water droplets and compressed gas, for example carbondioxide, can be release together so that the adiabatic cooling of theexpanding gas will freeze the water droplets into ice particles. Theresulting mixture of expanded gas and frozen particles can be mixed withair and/or with oxygen and other gases to produce the desired breathinggas mixture. Alternatively or in addition, the system may utilize othertypes of frozen particles, for example dry ice particles, to enhance theheat capacity of the breathing gas mixture.

The system is connected to a breathing mask 3 or tracheal tube for thepatient to breathe through. The frozen mist is carried into thepatient's lungs by the breathing gas. The ice particles melt within thepatient's lungs providing a high rate of heat transfer for cooling thelungs and the blood that flows through it. A heat transfer analysisoutlined below indicates the beneficial effect of the frozen mist on theheat transfer rate. The system is used in this manner until the desireddegree of hypothermia is achieved. Once hypothermia has been achieved,the rate of heat transfer can be reduced by reducing the quantity of iceparticles delivered and the temperature of the heat exchanger can beadjusted to maintain body temperature. One advantage of this embodimentof the system is that, because of the high heat transfer rate providedby the ice particles, an extremely low temperature will not be neededfor effective cooling of the patient thereby mitigating the risk offreezing damage to the patient's lungs. After the need for protectivehypothermia has passed, the system may be used for rewarming the patientto normothermia.

The amount of fluid that forms in the lungs from the melting of the iceparticles will be easily tolerated by the patient. An adult human withgood lung function can readily clear 1 liter per hour of fluid from thelungs through normal processes. Thus, a flow rate of ice particles inthe range of 0.25 to 1 liters per hour will be readily tolerated for anextended period of several hours. Higher flow rate of ice particles, upto 5 liters per hour, can be tolerated for shorter periods. If desired,positive pressure ventilation may be used to help drive the fluid fromthe lung passages into the surrounding tissue and from there into thebloodstream. In addition, diuretics or other medications to treatpulmonary edema may be administered to the patient to help eliminateexcess water if needed.

Anti-shivering agents and/or anti-thermoregulatory response agents maybe administered to the patient to assist in achieving the desired degreeof hypothermia. Alternatively or in addition, external warming, such aswith a warm air blanket or electric blanket, may be applied to reduceshivering while internal hypothermia is maintained. Regional heating ofselected portions of the patient's body may be used to control shiveringand/or to “trick” the body's thermoregulatory responses.

For increased heating or cooling effect, the apparatus and methodsdescribed herein can be used in combination with any known bodytemperature control systems, such as those described in the patentscited above. Alternatively or in addition, external heating or coolingcan be applied to augment the total heat transfer rate. Peripheralvasodilators and/or drugs that encourage perspiration may also beadministered to the patient to increase heat loss through the skin.

Analysis of the Effects of Adding Ice Particles to Breathing Mixture forControl of Patient Body Temperature

Baseline calculations and initial animal studies showed that cooling thebreathing mixture supplied to a patient will lower core body temperatureover time. Initial calculations for various mixtures of gases indicatethat the rate of heat transfer from the patient to the breathing mixturewill range from a low of about 10 Watts for the case of an He/O₂ mixtureat atmospheric pressure with an initial temperature of −30.degree. C.and a volumetric flow rate of 20 l/min, to a high of 117 Watts for thecase of an air/CO₂ breathing mixture at 2 atm pressure with an initialtemperature of −30.degree. C. and a volumetric flow rate of 100 l/min.

An analysis of the addition of ice particles to the mixture shows asignificant contribution to the heat transfer rate. The precise level offluid addition that is tolerable to the patient is as yet unknown, soanalyses were performed for ice particle addition rates ranging from0.25 to 5 liters/hour. The ice particles were assumed to be small insize, and to have an initial temperature of −30.degree. C. Heating ofthe ice particles was broken down into 3 steps: 1) heating the solid icefrom −30.degree. C. to 0.degree. C., 2) causing a solid-to-liquid phasechange at 0.degree. C., and 3) heating the liquid water from 0.degree.C. to 37.degree. C. For the various rates of addition of ice particles,the following heat transfer rates resulted:

TABLE 1 Rate of Ice Power to Power for Power to Particle Heat Ice Solid/Heat Liquid Total Addition from −30° C. to Liquid Phase from 0° C. toPower (liters/hour) 0° C. (W) Change (W) 37° C. (W) (W) 0.25 3.8 21.510.0 35.2 0.5 7.5 42.9 19.9 70.4 1 15.1 85.9 39.8 140.8 2.5 37.7 214.799.6 352.1 5 75.4 429.5 199.2 704.1

The calculations assume that the ice is mixed with air at atmosphericpressure, initially at −30.degree. C., with a volumetric flow rate of 20l/min.

FIG. 4 is a bar graph showing the heat removed from the body (Watts) asa function of the rate of ice particles added to the breathing mixture(1/hr).

FIG. 5 is a bar graph showing a more detailed breakdown of the heatremoved from the body (Watts) as a function of the rate of ice particlesadded to the breathing mixture (1/hr). The analysis shows that theaddition of ice particles will have a significant effect, and that themajority of the heat transfer is contributed by the phase change of theice to, liquid water, and that the smallest contribution comes from thepower required to heat the ice to 0.degree. C.

While the present invention has been described herein with respect tothe exemplary embodiments and the best mode for practicing theinvention, it will be apparent to one of ordinary skill in the art thatmany modifications, improvements and subcombinations of the variousembodiments, adaptations and variations can be made to the inventionwithout departing from the spirit and scope thereof.

1. A method for inducing therapeutic hypothermia in a patient,comprising: cooling a breathing gas mixture; injecting a fluid into thecooled breathing gas mixture to form a frozen mist of fine ice particlesin the cooled breathing gas mixture; delivering the cooled breathing gasmixture to an airway of the patient.
 2. The method of claim 1 whereinthe airway comprises a nasal airway of the patient.
 3. The method ofclaim 1 wherein the airway comprises a lung of the patient.
 4. Themethod of claim 1 wherein the airway comprises a lung and a nasal airwayof the patient.
 5. The method of claim 1 wherein the cooled breathinggas mixture is delivered to the airway of the patient through abreathing mask.
 6. The method of claim 1 further comprising the step ofinjecting a drug into the cooled breathing gas mixture.
 7. The method ofclaim 1 wherein fluid is injected into the cooled breathing gas mixturewith a fluid injector.
 8. The method of claim 1 wherein fluid isinjected into the cooled breathing gas mixture with an atomizer.
 9. Themethod of claim 1 wherein fluid is injected into the cooled breathinggas mixture with a nebulizer.
 10. A method for inducing therapeutichypothermia in a patient, comprising: forming a breathing gas mixture;cooling the breathing gas mixture; injecting a fluid into the cooledbreathing gas mixture to form a frozen mist of fine ice particles in thecooled breathing gas mixture; delivering the cooled breathing gasmixture to an airway of the patient.
 11. The method of claim 10 whereinthe airway comprises a nasal airway of the patient.
 12. The method ofclaim 10 wherein the airway comprises a lung of the patient.
 13. Themethod of claim 10 wherein the airway comprises a lung and a nasalairway of the patient.
 14. The method of claim 10 wherein the cooledbreathing gas mixture is delivered to the airway of the patient througha breathing mask.
 15. The method of claim 10 further comprising the stepof injecting a drug into the cooled breathing gas mixture.
 16. Themethod of claim 10 wherein fluid is injected into the cooled breathinggas mixture with a fluid injector.
 17. The method of claim 10 whereinfluid is injected into the cooled breathing gas mixture with anatomizer.
 18. The method of claim 10 wherein fluid is injected into thecooled breathing gas mixture with a nebulizer.
 19. The method of claim10 wherein the forming step comprises forming the breathing gas mixturefrom a gas mixture that includes oxygen and a gas with a high heatcapacity.
 20. The method of claim 10 wherein the forming step comprisesforming the breathing gas mixture from a gas mixture that includesoxygen and helium.
 21. The method of claim 10 wherein the forming stepcomprises forming the breathing gas mixture from a gas mixture thatincludes oxygen and carbon dioxide.
 22. The method of claim 10 whereinthe forming step comprises forming the breathing gas mixture from a gasmixture that includes sulfur hexafluoride.
 23. The method of claim 10further comprising pressurizing the breathing gas mixture.